RU1788919C - Bimetallic wire for high-temperature resistor strain gauge elements - Google Patents

Abstract

Usage: in the development of materials for the output conductors of high temperature strain gages. SUBSTANCE: bimetallic wire contains a conductive central conductor of the material of the following chemical composition, May. %: nickel 43 ... 45, silicon 0.001 - 0.005: impurities 0.1 ... 0.2; copper the rest. The bimetallic wire also contains a heat-resistant sheath made of an alloy of the following chemical composition, May. %: chrome 19 ... 23; silicon 0.4 to 1.5; zirconium 0.03 ... 0.05; nickel rest. The volume fraction of the central conductor in the bimetallic wire is 48 ... 60%. This bimetallic wire provides materials with high electrical conductivity, heat resistance and low temperature coefficient of electrical resistance. 3 tablets, 4 ill. ate

Description

The invention relates to metallurgy (electrical engineering), and in particular, to the development of materials for the output conductors of high temperature strain gages (VP VTR).

In the practice of high temperature strain gauge measurements, the most important requirements are to ensure accuracy and stability of measurements with sufficient reliability of strain gauge systems over a wide range of operating temperatures. Elements of tensometric systems must have sufficient strength and ductility.

All of these requirements are fully met for newly developed materials for elements of measuring systems, in particular, VTR lead-out conductors during their development and in the process of their further improvement.

The measurement accuracy is influenced by the appearance of an uninformative component of the output signal (error) when heating, which is mainly associated with a change in the electrical resistance of the two output conductors of the strain gauge. And this change is proportional to the values of electrical resistivity p and thermal coefficient of resistance a of the material of the output conductors.

The stability of the results of tensometric measurements is related to the stability over time of the structural-phase state of the metal of the lead conductors and also depends on the nature of the physicochemical bonds VI

with

00 s

YU

with

actions in the boundary layers of the metal of these conductors when heated to high operating temperatures.

The reliability of VTR lead-out conductors is determined by long-term heat resistance without noticeable oxidation, embrittlement with a sufficient level of mechanical properties - strength and ductility (to avoid failure due to rupture and resistance to bending and torsional loads).

The need to develop an electrically conductive material for the needs of the electrical industry and instrumentation with sufficiently high heat resistance and at the same time with low electrical resistivity led to the creation of bimetallic materials. They consist of a highly conductive core (center conductor) enclosed in a heat-resistant sheath. The core material is copper and silver. Nickel, stainless steel, nickel-chromium-to-belt alloys are used for the sheath.

The closest in technical essence and the achieved result is a bimetallic wire containing an electrically conductive central conductor and a heat-resistant sheath.

The electrically conductive composite material is made in the form of a bimetallic wire with a central conductor of copper (GOST 2112-79) in a heat-resistant sheath made of 1X18H10T steel (GOST5632-61) or an alloy based on nickel of the following chemical composition (wt.%):

Cr 10.0; Si 2.0; Ni stop

To ensure reliable protection against oxidation, the cross-section of the sheath is 25-40%, and the cross-section of the central wire, respectively, is 75-60% of the total cross-section of the wire. Known material is made in the form of a wire with a diameter of 0.20 mm.

As a generalized criterion for assessing the operational properties of existing and newly created electrically conductive materials for output conductors and VTR communication lines, the quantity a (t) was chosen:

d (t) p a D t,

where p is the value of the electrical resistivity of the material at 20 ° C, μOhm-m;

a - thermal coefficient of electrical resistance of the material in the temperature range from room to working,

At is the temperature difference between working and room, taken at 20 ° С.

Table 1 shows the electrical properties of the wire from the above material after quenching from 900 ° C in water, as well as the properties of the wire of nichrome X20H80 alloy (GOST 12766-67) used for the manufacture of lead conductors in commercially available strain gauges.

10 The disadvantage of the known materials listed in the table is the high values of electrical resistivity and temperature coefficient of electrical resistance (parameter e) and relatively

15 low upper limit of the operating temperature of operation, which significantly limits the field of application of VTR communication lines in measurement systems.

The aim of the invention is to improve the quality of the bimetallic wire by increasing its operational properties, namely, simultaneously reducing the electrical resistivity and temperature coefficient

25 electrical resistance, increasing heat resistance, service life, strength and ductility of the material. This allows measuring systems, including elements of VTR communication lines, to service objects

30 measurements under conditions of simultaneous exposure to various kinds of thermomechanical loads.

This goal is achieved by the fact that the electrical conductive central conductor 35 is made of the material of the following chemical composition (May.%): Nickel 43 - 45; silicon 0.001-0.05; impurities 0.1 - 0.2; the rest is copper, and the heat-resistant shell is made of an alloy of the following chemical

40 composition (May.%): Chromium 19-23; silicon 0.4 to 1.5; zirconium 0.03 - 0.05; nickel rest.

In this case, the volume fraction of the central conductor in the bimetallic wire

45 is 48-60%.

In a theoretical study of the issue, the problem of finding a composite material in the form of a wire of small diameter with the smallest possible

50 a (i.e., with a minimum value of the generalized parameter b) in the entire range of positive operating temperatures, up to high (900 ° C). The required level of electrical properties must be combined with high heat resistance, strength and ductility of the material.

The main fundamental difficulty in creating this kind of material is related to the inverse relationship between

“For most metals and alloys (Mui rule). Acceptable materials for the magnitude of this parameter are nickel-copper alloys. However, they have an advantage over nickel-chromium alloys only up to 400 ° C and cannot be considered promising due to their low heat resistance.

Assessing the insufficient performance characteristics of existing monometallic materials, composite materials in the form of a bimetallic wire based on nickel-chromium / nickel-copper systems were selected as objects of study. In this case, the positive and negative nature of the temperature dependence,

In FIG. 1 shows the relative change in the electrical resistance of alloys-components of composite materials when heated in the range of 20 - 900 ° C; in FIG. 2 - relative change in electrical resistance of TMK during heating in the range of 20 - 900 ° C; in FIG. 3 - temperature dependence of electrical resistance of a bimetallic wire from a composite: sheath (nickel-chromium alloy) / central conductor (nickel-copper alloy); volume fraction of the central conductor 48 - 60%; in FIG. 4 - the region of existence (see Tables 2 and 3) of the optimal values of the main operating characteristics of the enclosed bimetallic wire in a diameter of 0.15 mm at a test temperature of 900 ° C.

In the drawings in the symbols:

in FIG. 1 1 - nickel-chromium-based alloy (20); 2 - nickel-copper based alloy; 3-alloy type nickel-chromium (10%); D R / R20 is the relative change in electrical resistance when heated from 20 ° C to a temperature of T ° C;

in FIG. 2 1 - nickel-chromium alloy / nickel-copper alloy (50%); 2 - nickel-chromium alloy (10%) / nickel-copper alloy (80%);

in FIG. 4 S - volume fraction of the central conductor; t eoo heat resistance of bimetallic wire at 900 ° С;

d (T) is a generalized parameter by the formula (1).

Table 2 shows the chemical composition of the alloys that make up the initial composite billet.

Table 3 gives the results of a study of the operational properties of the manufactured bimetallic wire with a diameter of 0.15 mm.

Based on the inverse interdependence of the electrical resistivity of TCS a in the vast majority of alloys, to solve the problem, a search was made for a composite material that combines components with positive and negative dependences of electrical resistance on temperature. In this case, one of the components must have high electrical conductivity or, in other words, low electrical resistance. During the study, analytical expressions were used to calculate / Ekm composite

materials depending on the corresponding characteristics of their constituents

twenty

PI-P2

(-x)

(2)

where p km is the electrical resistivity of bimetals;

p - electrical resistivity of the components 1 and 2 of bimetal, respectively, determined by the formula:

/31.2 p 1.2 (20) (1+ a A t),

x is the volume fraction of the bimetal component 2.

In creating the majority of bimetallic materials for use in electrical engineering, only two technical problems were solved: high electrical conductivity and a sufficient (500-600 ° C) level of heat resistance were achieved.

The goal of achieving a low level of TCS was not actually set. Thus, for bimetal, steel 4X18H10T / copper, the parameter pa in the range of 20-600 ° C is even higher than that of the monometallic alloy X20H80 due to the high value of a for copper.

For the TCS composite, there is also an approximate calculation formula:

and,

 "I

+ "2 (1-x), (4)

where a km, ai and "2 - TCS of the composite and its components.

At the beginning of the study, due to the lack of initial data and the complexity of obtaining composite samples for the selection of experimental compositions of composites, electrical properties were simulated on thermomacrocomposite samples (TMC), representing a pair of twisted wire

conductors made of alloys identical to the components of bimetals. These model samples are called thermoelectrode, they are quite close analogues of compositions with continuous contact of component elements. In this case, the temperature dependences of the electrical resistance (t) TMK approximately reproduce the corresponding dependences for bimetals of the same compositions.

The analysis of FIG. 1 and 2 of the temperature dependences of the electrical resistance of TMC and alloys of components will explain the methodology for selecting (by calculation and experiment) the compositions of the composites according to the parameters p and a to obtain them in the form of a bimetallic wire in this case for systems: Ni-Cr alloy / Ni alloy - Cu (50 - 60), alloy НХ9.5 / alloy Ni - Cu (50 - 60 vol.%). Thus, in FIG. 1 and 2 it can be seen that the solution to the problem is determined on a compensation basis of the characteristics of the alloys constituting the bimetal wire. In FIG. H shows the dependence of the change in electrical resistance on temperature for a bimetallic wire for VTR, made according to the results of the survey,

As a result of theoretical calculations of the heat resistance characteristics of the found composite at 900 ° C, its oxidation characteristics at this temperature for 4 hours were compared with the oxidation characteristics of the reference heat-resistant alloy, nichrome. In terms of heat resistance, the composite found is inferior to nichrome by only 15 - 20%.

Thus, the solution to the problem of minimizing the considered electrical properties of bimetal with a simultaneous maximum increase in the operating temperature and the service life of the bimetallic conductor made it possible to determine the second part of the general problem — the optimal ratio of the volume parts of the central conductor and the composite shell with the known chemical composition of the composites.

To obtain a specific material, nickel-chromium and nickel-copper alloys were smelted, the chemical compositions of which are given in Table. 2. Smelting was carried out in a vacuum induction furnace with a 25 kg charge, followed by casting into 1 ingot. In total 11 smelting ingots were smelted. Of these, five (I ... V) are from the alloy-shell component of the bimetal - the Ni-Cr alloy. Five more smelting ingots (VI ... X) are made of an alloy constituting the central conductor of the bimetal - an alloy of Ni - Cu. The last heat (XI) was smelted from a monometall - nichrome - to determine the quantitative ratio of its service characteristics with the bimetal under consideration.

The ingots from the Ni - Cr alloy, after heating at 1250 ° C, were forged on a blacksmith's hammer with c. n. h. 500 kg on rods with a diameter of 45 mm. After heating at 1100 ° С, Ni - Cu alloy ingots forged exactly the same on rods with a diameter of 34 mm. After machining and assembly, composite billets with an outer diameter of 39.5 mm were obtained from the forged bars, having axial central cores of various diameters, which determine different ratios of the cross-sectional area of the bimetallic billet in the initial state.

To determine the optimum chemical composition and the ratio of the areas of the central core to the total wire cross section of the bimetal to be attached, composite billets were made.

designated I / VI, II / VIII, lil / VIII, IV / IX and V / X, with a central core volume content in the total billet volume of 41.1 ... 67.3% with a variation in the chemical composition of the components of the composite billet

(table 2).

The prepared billets were heated to 1230–1260 ° С and subjected to hot extrusion onto bars of 6.0–8.0 mm. After the operation of quenching in water from a temperature of 1050 ° C

and pickling from rods by cold drawing, a bimetallic wire of 0.15 mm was obtained. All specified ratios between the total cross-section of the wire and the inner (central) conductor

after hot and cold deformation, they persisted.

Table 3 shows the properties of the bimetallic wire after the final heat treatment at 1050 ° C in

passage hydrogen furnace.

High electrical conductivity in combination with a low thermal coefficient of electrical resistance and high heat resistance provide lower values of the d (t) parameter for a bimetallic wire than that of known bimetals and nichrome monometallic alloys, and in a wider range of operating temperatures (20 - 900 ° C) than established for the prototype and other possible composites considered in the above study (Figs. 1 and 2).

The aforementioned advantages make it possible to use the developed material for the manufacture of lead-out conductors of promising high-temperature strain gages, as well as for converters for various purposes, requiring the use of materials with high electrical conductivity, heat resistance, and low coefficient of electrical resistance.

From the data table. 2 and 3, the optimal options for the manufacture of composite wire are 1, 2, 3, 6, and 7 providing the best physicomechanical operational properties of the embedded wire.

Claims (1)

SUMMARY OF THE INVENTION A bimetallic wire for elements of high temperature strain gauges comprising an electrically conductive center conductor and a heat resistant a sheath, characterized in that, in order to improve the quality of the bimetallic wire by increasing its operational properties, the conductive central conductor is made of the material of the following chemical composition, wt.%: Nickel 43-45 Silicon 0.001-0.005 Impurities 0.1-0.2 Copper The rest and heat-resistant casing are made of an alloy of the following chemical composition, May. %: Chromium 19-23 Silicon 0.4-1.5 Zirconium 0.03 - 0.05 Nickel The rest with the volume fraction of the central conductor in the bimetallic wire being 48-60%. The main operational parameters of the known mono- and bimetallic conductors The chemical composition of the alloys constituting the initial composite billet before hot extrusion Table 1 Tables2 The result of the study of the operational properties of the embedded ometallic wire with a diameter of 0.15 mm -4.0 20 203 4QQ 600 WO- 1 ° opt . . -. ° C Table3 qbas.i 4000  T FIG. 2 - Tt ec , 3 FIG. AND.